Requests on information services are rapidly increased as is observed in generalization of information communication services, appearance of a variety of multimedia services, advent of high quality services, and the like. A variety of wireless communication techniques are developed in various fields to satisfy such requests.
Next generation wireless communication systems should be able to transmit high-quality high-volume multimedia data in a high speed using limited frequency resources. In order to make it possible through a radio channel with limited bandwidth, interference among symbols and frequency selective fading should be overcome while maximizing spectral efficiency. Multiple input multiple output (MIMO) is one of techniques that can improve communication capacity and performance without allocating additional frequencies or increasing power.
MIMO is a technique developed to increase capacity or improve performance using multiple antennas in a transmitter and/or a receiver of a wireless communication system. The MIMO technique does not depend on a single antenna path in order to receive one whole message, but applies a technique that gathers fragmented data segments received through multiple antennas and reproduces a message. Data transmission speed may be improved within a specific range, or a system range may be increased for a specific data transmission speed. If the number of transmit and receives antennas is increased at the same time, theoretically, channel transmission capacity is increased in proportion to the number of the antennas, and thus spectral efficiency may be improved.
The MIMO technique may be categorized into a spatial diversity method for enhancing transmission reliability using diverse channel paths and a spatial multiplexing method for improving a transmission ratio by simultaneously transmitting a plurality of data streams. Recently, studies on a method for appropriately combining these two methods to obtain adequate advantages of the methods are actively conducted.
The spatial diversity method is divided into a space-time block code (STBC) method and a space-time trellis code (STTC) method for simultaneously enhancing diversity gain and coding gain. Generally, the STTC method shows excellent performance in improving a bit error rate and has a high code creation freedom degree, whereas the STBC method has a lower operation complexity. Spatial diversity gain may be obtained as much as an amount corresponding to a multiplication of the number of transmit antennas and the number of receive antennas.
The spatial multiplexing is a method for transmitting different data streams through different transmit antennas. Since mutual interference occurs among simultaneously transmitted data streams, the receiver should process the streams after canceling the interference using a proper signal processing technique. The receiver may be divided into an ML (Maximum Likelihood) receiver, ZF (Zero-Forcing) receiver, MMSE (Minimum Mean Square Error) receiver, BLAST (Bell Labs Layered Space Time) receiver, and the like depending on a method used to cancel interferences.
When only spatial diversity gain is taken, performance improvement gain depending on increase of diversity order may be gradually saturated. When only spatial multiplexing gain is taken, transmission reliability may be lowered. Methods for obtaining both of the gains while solving the problems have been studied, which include Double-STTD, Space-Time Bit-Interleaved Coded Modulation, and the like.
On the other hand, a fading channel is one of the major causes that invite degradation of system performance of a wireless communication system. A channel gain value is changed depending on time, frequency, and space, and the performance is seriously degraded as the channel gain value is lowered. Diversity, which is one of methods capable of overcoming fading, uses the fact that probability of a plurality of independent channels to simultaneously have a low channel gain value is very low. One method of a variety of diversity methods is multi-user diversity. When there is a plurality of users in a cell, channel gain values of each users are independent from one another in probability, and thus probability of all the users to have a low gain value is very low. According to information theory, if the base station has sufficient transmission power when there is a plurality of users in a cell, total channel capacity is maximized when all channels are assigned to a user having the highest channel gain value.
The multi-user diversity may be divided into three methods. First, time multi-user diversity is a method of assigning a channel to a user having the highest gain value each time when the channel varies over time. Second, frequency multi-user diversity is a method of assigning a subcarrier to a user having the highest gain value within each frequency band in a multiple carrier system such as an OFDM (Orthogonal Frequency Division Multiplexing) system. If the channel changes very slowly in a system that does not use multiple carriers, a user having the highest gain value will exclusively use the channel for an extended period of time, and thus the other users cannot communicate. In this case, it needs to induce change of the channel in order to use the multi-user diversity. Third, spatial multi-user diversity is a method of using different channel gain values of users depending on a space, and random beamforming is an example implementing the method. The random beamforming is also referred to as opportunistic beamforming, which performs beamforming with an arbitrary weight value using multiple antennas in a transmitter to induce change of the channel.
In conventional wireless communication system, a transmitter transmits an information stream after coding the information stream using a forward error correction code so that a receiver may correct an error. The receiver demodulates a receive signal, decodes the demodulated signal using the forward error correction code, and restores the information stream.
All forward error correction codes have a maximum correction limit in correcting channel errors. That is, if a receive signal has errors exceeding the limit of a corresponding forward error correction code, the receiver cannot decode the receive signal. Accordingly, the receiver needs a criterion for determining whether there is an error in the decoded information. In addition to an error correction coding procedure, an error detection code is needed to detect errors. Generally, CRC (Cyclic Redundancy Code) is widely used as such an error detection code.
CRC is not an error correction method, but one of coding methods used for detecting errors. Generally, an information stream is coded using CRC, and a forward error correction code is applied to the CRC coded information. A unit of information stream that is coded by applying the CRC and the forward error correction code is referred to as a codeword. When a plurality of codewords are overlappingly transmitted, performance may be improved by using a receiver of an interference cancellation method.
A structure of the interference cancellation is briefly described below. A first receive signal is demodulated/decoded from the whole receive signal in which a plurality of receive signals are overlapped, and then the first receive signal is removed from the whole receive signal. A second receive signal is demodulated/decoded using the signal remaining after the first receive signal is removed from the whole receive signal. Demodulation/decoding of a third receive is performed using the signal remaining after the first and second receive signals are removed from the whole receive signal. Signals following the third receive signal are demodulated/decoded by repeating the processes described above. In order to use the interference cancellation method, the demodulated/decoded signals extracted from the receive signal should not have an error. If there is an error, an error propagation phenomenon occurs which subsequently gives negative affects when all signals thereafter are demodulated/decoded.
The interference cancellation technique may be also used in the multiple antenna technique. In order to use such an interference cancellation technique, a plurality of codewords should be overlappingly transmitted throughout multiple antennas. That is, when the spatial multiplexing technique is used, the interference cancellation technique may be used while detecting each codeword.
However, as described above, in order to minimize the error propagation phenomenon that occurs when interference is cancelled, it is determined whether there is an error in the extracted demodulated/decoded signals, and then interference should be selectively canceled. CRC described above is a practical means for determining whether there is an error in the transmitted information. Generally, a unit of information that passes through the CRC coding and is distinguished from the other information may be referred to as a codeword. That is, there should be several pieces of transmission information and a plurality of codewords in order to use the interference cancellation technique.
There is a variety of feedback information needed to efficiently operate the MIMO system.
First, a receiver should inform a transmitter of information on channel quality. A channel quality indicator (CQI) represents information on the channel quality. The CQI may be transmitted in various format. For example, the CQI may be in the form of signal-to-interference plus noise (SINR), modulation and coding scheme (MCS), and the like. The MCS includes a modulation scheme and a coding scheme. It is general that at least one CQI is needed per codeword. As the number of needed CQIs is increased since the MIMO system uses multiple channels through multiple antennas, that of codewords is increased. Accordingly, the amount of feedback information is increased in proportion to the CQI.
Second, since the MIMO system can transmit multiple data stream, feedback information for informing how many independent data streams can be transmitted at the current moment of transmission should be transmitted. This is referred to as a rank. Generally, the rank is the number of available data streams (or codewords) that can be transmitted at a moment of transmission. Since one CQI is transmitted per codeword, two CQI is transmitted at rank=2. The rank is determined depending on a combination of transmit and receive antennas. In a system having M transmit antennas and N receive antennas, the maximum rank is min(M, N).
Third, in a MIMO system that uses precoding, feedback information needs to includes information on a precoding matrix that is most appropriate to the current channel state. Although a precoding matrix described here, it can be a vector form as well as a matrix form. A precoding matrix indicator (PMI) represents an index of the selected precoding matrix from a codebook which includes a plurality of precoding matrix.
As described above, when the number of codewords is increased in the MIMO system, the number of CQIs is proportionally increased. If one CQI is configured with five bits and there are two codewords, total CQIs are configured with ten bits. Since every user who should inform a channel state transmits the CQI, the CQI occupies a large portion from the view point of total radio resources. Accordingly, it is desirable to reduce the amount of feedback information. Furthermore, since the CQI has a different value for each subband, in a system that uses frequency selective channel scheduling such as an orthogonal frequency division multiple access (OFDMA) system, the amount of CQIs may be greatly increased.
The maximum rank is four if four transmit antennas is considered. Accordingly, two information bits are needed to transmit the rank. In the case the PMI, if the codebook has N precoding matrixes, log2(N) information bits are needed to transmit the PMI. Considering the frequency selective channel scheduling, since the PMI may be different for each subband, the amount of radio resources for the PMI may be huge.
A large amount of feedback information needs to operate a MIMO system. This means that a large amount of radio resources is allocated to a uplink control channel or a uplink data channel to transmit the feedback information.